1. Field of the Invention
In general, this invention relates to electrical sensing and measuring, more specifically, to determining movement of objects carrying charges.
2. Description of the Prior Art
Movements of objects are detected by employing either active or passive motion detectors. Active detectors radiate test signals to the environment (like ultrasound, microwaves, infrared light, etc.) and detect either reflected signal or disturbances in the radiation pattern due to object movement. Passive detectors do not radiate any signal and detect whatever is naturally radiated by surroundings toward the sensor, like thermal radiation. The most commonly used of the latter are passive infrared (PIR) detectors. Such detectors are disclosed for instance by Schwartz (U.S. Pat. Nos. 3,760,399 and Re. 29,082), Smith et al. (U.S. Pat. No. 4,379,971), Cohen (U.S. Pat. No. 3,809,920), Fraden (U.S. Pat. No. 4,769,545) and others. Apart from many advantages, the PIR detectors have their limitations, such as reduced sensitivity when temperatures of an object and surroundings become equal or close to each other, they require focusing optical components, are sensitive to piezo-electric interference, require direct vision of an object.
Active detectors are usually large, consume substantial amount of energy, generate mutual interference and are subject to simple countermeasures. Passive detectors are more economical although their operation depends on presence in the sensor's vicinity of some kind of field related to a moving object. Thermal radiation, which is detected by PIR is one example. Another field which might be associated with a moving object is electrostatic field.
There are sensors known in the prior art which measure variable electric charges. All these sensors require use of high input impedance amplifiers as exemplified by the U.S. patents issued to Gathman et al (U.S. Pat. No. 3,644,828) and Andrus et al. (U.S. Pat. No. 4,063,154).
A variety of electrodes have been proposed to detect electrostatic field. A U.S. patent issued to Blitshteyn et al. (U.S. Pat. No. 4,529,940) teaches an application of a circumferential electrode with a rotating cylindrical chopper, while the U.S. patent issued to Polukhina et al. (U.S. Pat. No. 4,041,375) describes an areal type electrode which detects electromagnetic signals radiated from discharged static electricity.